A pressure-sensitive adhesive should be soft so that it can develop sufficient interfacial contact area with a substrate on application of very light pressure. It must, however, also be strong enough to resist debonding failure. Conventionally, this rate-dependent viscoelastic property of pressure-sensitive adhesives was achieved by blending tackifying resins with a rubber. More recently, there has been an increased interest in using polyacrylates among other polymers as pressure-sensitive adhesives for their superior weathering and aging properties. There are, however, some disadvantages in the use of such acrylic pressure-sensitive adhesives. The materials cost of the acrylates is higher than the conventional rubber-based adhesive, and tack performance is normally lower. In addition, it is difficult to find good tackifiers that are commercially available for acrylic adhesives. When a porous material such as paper is used as the facestock, many compounded acrylic adhesives are prone to have certain components "bleed" out and stain the facestock.
In 1964 Wetzel presented a model of a compoundable pressure-sensitive adhesive based on a resin and a sticky low-molecular-weight rubber in which a sticky low-molecular-weight tackifier was dispersed in the bulk of the base resin. Solid tackifiers have since been conventionally employed to induce pressure-sensitive characteristics to a base polymer. The effect of tackifier addition is shown in FIG. 1, attached hereto.